1. Field of the Invention
The invention pertains to microwave proximity sensors for detecting the presence, absence or motion of an object of interest by means of generating and transmitting microwaves and by receiving and processing waves reflected from such object.
2. Description of the Prior Art
Microwave proximity sensors are devices which detect the presence, absence or motion of reflective objects of interest, such as a human body. The basis of their operation is that certain objects, including the human body, will reflect radio waves of the appropriate frequencies. A body in the vicinity of a microwave radio transmitter, for example, will reflect a portion of the transmitter's output back toward the transmitter, thus generating a standing wave. If the object should move, the standing wave pattern will shift, thereby causing a change in the field intensity in the vicinity of the transmitter. These characteristics have been utilized to provide equipment for detecting the presence, absence or motion of human bodies, for example, in such applications as security equipment.
Existing devices generally comprise a microwave oscillator, a transmitting antenna, a receiving antenna and a detector. The output of the oscillator is coupled to the transmitting antenna, which radiates the output in the desired direction. The receiving antenna is so placed that it can receive the transmitted signals directly and any reflected signal, and is connected to the detector. Changes in the standing wave pattern produce changes in the output of the receiving antenna which are recognized by the detector, thus providing sensing of changes in reflective bodies in the surroundings of the device.
Existing devices generally utilize solid-state devices to generate the microwave signal, which determines operation frequency and size and shape of transmitting and receiving antennas. Drawbacks of certain existing devices have been related to their complexity and cost, particularly of specialized microwave components, and further by the need for individual adjustment to match output frequency of the oscillator to the resonant frequency of the associated antenna. Additional drawbacks have been size and power consumption of existing proximity sensors, which have hindered their use in applications where compactness or portability of the sensor or power source is a significant consideration, as in the case of toys.
Thus, there has been a felt but unfulfilled need for a microwave sensor exhibiting simplicity and economy in structure and operation.